Tests on exterior building materials in other high-rise buildings conducted after the devastating west London fire has shown that some of the panels submitted were combustible, British Prime Minister Theresa May said Thursday.

May told the House of Commons that authorities submitted cladding on similar buildings following the Grenfell Tower fire on June 14 that killed at least 79 people. The aluminum composite material is being studied to see if it contributed to the quick spread of the blaze, which consumed the 24-story building in less than an hour.

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“Shortly before I came to the chamber, I was informed that a number of these tests have come back as combustible,” she said. “The relevant local authorities and local fire services have been informed, and, as I speak, they are taking all possible steps to ensure buildings are safe and to inform affected residents.”

Britain’s opposition leader, Jeremy Corbyn, called for urgent checks on around 4,000 buildings as Britain comes to grips with the potential ramifications of the disaster.

“At least 79 people are dead — it is both a tragedy and an outrage because every single one of those deaths could and should have been avoided,” Corbyn said.

May has apologized for mistakes made in dealing with the aftermath of Grenfell Tower tragedy and promised that “no stone will be left unturned” in the inquiry.

“For any guilty parties there will be nowhere to hide,” she said.

May’s comments came following the resignation of the local administrator in the west London community devastated by the fire after government officials criticized the speed of the response to the tragedy.

Nicholas Holgate, chief executive of the Kensington and Chelsea council, had come under intense pressure in the wake of last week’s Grenfell Tower blaze. The first few days after inferno were marked by chaos on the ground as local authorities struggled to deal with the scope of the disaster.

Residents who survived the tower blaze lost everything, only to get little help or information on how they’d get back on their feet.